1. Field of the Invention
This invention relates to a concentration method of an aqueous acrylamide (hereinafter abbreviated as "AAM") solution, and especially to a concentration method featuring separation of unreacted acrylonitrile (hereinafter abbreviated as "AN") and excess water from a synthesis reaction mixture of AAM available through a hydration reaction from AN by a catalyst or a microorganism. This invention is also concerned with a concentration method of a dilute aqueous AAM solution which is substantially free of AN. More specifically, this invention pertains to an improved concentration method which makes it possible to obtain a dense aqueous AAM solution of good quality while preventing polymerization of AAM without addition of any polymerization inhibitor other than air.
2. Description of the Related Art
In the form of AAM-base polymers, AAM has been used for many years as papermaking chemicals, flocculants, oil recovery agents and the like. It also has a wide range of utility as a starting co-monomer for various polymers. For such utility, AAM was produced by the so-called sulfuric acid process previously. In recent years, processes making use of metal-copper-based catalysts or microorganisms have been developed and are industrially practiced these days. AAM is generally furnished as power or an aqueous solution to the market.
Upon production of an aqueous AAM solution through a catalytic hydration reaction of AN by a metal-copper-based catalyst, problems such as a reduction in the reaction velocity and by-production of impurities arise if the AN concentration in a feed solution is raised excessively or the conversion percentage in the reaction is increased too much. It is therefore the common practice to leave unreacted AN in a synthesis reaction mixture so that the concentration of AAM is limited to 20 to 40% or so. To convert the thus-obtained aqueous AAM solution into the commercial form of powder or an aqueous solution, it is necessary to remove unreacted AN and excess water from the synthesis reaction mixture so that the synthesis reaction mixture is concentrated to an aqueous AAM solution of a specified concentration. In some instances, it may also become necessary to concentrate a dilute aqueous AAM solution which is substantially free of AN.
In the course of such concentration, AAM is prone to polymerization, and an attempt to achieve a specified concentration tends to result in a product with AN still remaining therein without sufficient removal or in a low-quality product with AAM polymer mixed in a large amount. Concerning the concentration of an aqueous AAM solution, several methods have been proposed to date. In the case of concentration from a three-component system of AN-AAM-water, it is generally required to lower the concentration of remaining AN to 0.1% or lower in a concentrated solution. To this end, it is the common practice to use a distillation apparatus composed of a fractionation unit and a heat evaporation unit.
According to Japanese Patent Laid-Open No. 62713/1973, a distillation apparatus composed of a fractionation unit and a heating unit is employed. A solution to be distilled is caused to pass in the presence of a small amount of AN, which acts to suppress polymerization of AAM, through the fractionation unit. The solution is next caused to promptly evaporate in the heating unit, for example, by using a centrifugal-film evaporator, so that the small amount of AN is removed. This method is stated to be free from adhering and accumulation of gel-like polymer and also to permit sufficient removal of AN and provision of an aqueous AAM solution of a specified concentration, said solution containing polymer only in a trace amount.
Japanese Patent Publication No. 26587/1982 discloses a concentration method which makes no use of a fractionation tower for the removal of AN because such a fractionation tower induces polymerization of adhered AAM. Described specifically, subsequent to flash evaporation of a reaction mixture of AAM synthesis, said reaction mixture containing AN, AAM and water, the thus-evaporated reaction mixture is concentrated using a centrifugal-film evaporator in which the reaction mixture and evaporating steam are caused to flow countercurrently. This method is stated to permit sufficient removal of AN and also provision of an aqueous AAM solution of high concentration without occurrence of polymerization.
Concentration methods making use of oxygen-containing gases, such as air, as stabilizers are also disclosed. Japanese Patent Publication No. 27898/1980 discloses a concentration method. According to this method, a dilute aqueous AAM solution is fed to a concentration system composed of an oxygen gas absorption unit, a heating unit and a reduced-pressure evaporation unit. While recirculating a dense aqueous AAM solution in an amount 10 to 500 times as much as the thus-fed aqueous AAM solution through the units of the concentration system, oxygen is caused to dissolve in the aqueous solution in the oxygen gas absorption unit, the aqueous solution is heated to 20 to 60.degree. C. in the heating unit to maintain a temperature difference between the feed solution to the evaporation unit and an effluent from the evaporation unit within a range of from 1 to 20.degree. C., and water is then caused to evaporate off under reduced pressure.
Further, Japanese Patent Publication No. 28777/1977 discloses that the stability of a dilute aqueous AAM solution can be improved by adding at least one of the hydroxides, carbonates and bicarbonates of alkali metals.
Japanese Patent Laid-Open No. 78293/1993 discloses a method for concentrating a dilute aqueous AAM solution. This method makes use of a heating unit and a gas-liquid separator. The heating unit is maintained in a wet state, and the aqueous AAM solution is heated together with an oxygen-containing gas so that water is absorbed in the oxygen-containing gas.
Japanese Patent Laid-Open No. 62717/1973 discloses that an aqueous AAM solution of a specified concentration, which will develop no quality problem under high temperature and atmospheric pressure, can be obtained by adequately bringing air in an amount as much as 0.1 times or more in terms of moles of the water to be evaporated into contact with an aqueous AAM solution upon concentration of the last-mentioned aqueous AAM solution.
In contrast to the above-mentioned methods which make use of air as a stabilizer, methods featuring concentration in a non-oxidizing atmosphere are disclosed in Japanese Patent Publication No. 27048/1980 and Japanese Patent Publication No. 12344/1985.
Japanese Patent Laid-Open No. 106420/1979 discloses a concentration method in which attributing polymerization to oxygen which flows into a concentration apparatus, nitrogen monoxide is added in an amount of 4 moles or greater per mole of oxygen flowing into the apparatus so that oxygen-induced polymerization can be prevented.
According to the present inventors' knowledge and finding, polymerization can be prevented for the first time by allowing copper ions, which serve as a polymerization inhibitor, to exist at a high concentration, for example, of 100 ppm or more based on AAM upon conducting concentration in an non-oxidizing atmosphere without introduction of air as disclosed in Japanese Patent Publication No. 62717/1973 or Japanese patent Publication No. 12344/1985. Copper ions are removed in a purification step, but the use of copper ions at such a high concentration results in a substantial load to the purification step and is thus not preferred.
The method disclosed in Japanese Patent Laid-Open No. 106420/1979, which makes use of nitrogen monoxide as a polymerization inhibitor, is an effective method for the prevention of polymerization in a concentration step. However, when an AAM product concentrated and purified in accordance with this method was polymerized and the quality of the AAM product was evaluated based on the water solubility and ultimate molecular weight of the polymer, the results were not preferred. Moreover, use of nitrogen monoxide on a commercial scale is not practical for its high cost, toxicity and the like.
Methods for reducing the concentration of AN to a level of 0.1% or lower in a three-component system of AN-AAM-water are disclosed in Japanese Patent Laid-Open No. 62713/1973 and Japanese Patent Publication No. 26587/1982. These publications make no mention about polymerization inhibitors and therefore, these methods are considered to be effective only when a polymerization inhibitor such as copper ions exists at a high concentration.
The prevention of polymerization becomes very difficult when a solution with a polymerization inhibitor such as copper ions contained only at a low concentration is subjected to concentration by an apparatus composed of a heating unit and an AN-separating fractionation unit. Introduction of an oxygen-containing gas such as air has certain effects for the prevention of polymerization but, as the operation goes on, popcorn polymers adhere and accumulate especially in the fractionation unit, thereby making it difficult to continue the operation further.
Since polymers tend to occur and adhere in an AN-separating fractionation unit as described above, Japanese Patent Publication No. 26587/1982 discloses a concentration method which makes no use of a fractionation tower, that is, which performs concentration by using a centrifugal-film evaporator where a mixed solution and evaporating steam flow countercurrently. In this case, however, the separation of AN tends to become insufficient.
Concentration of a two-component system of AAM-water, which does not require separation of AN, is disclosed in Japanese Patent Publication No. 27898/1980, Japanese Patent Publication No. 28777/1977 and Japanese Patent Laid-Open No. 78293/1993. This system does not require any fractionation unit so that the concentration apparatus is composed of a heating unit and a reduced-pressure evaporation unit. In this respect, the methods are advantageous for the prevention of polymerization. According to the inventors' finding, however, unless a polymerization inhibitor such as copper ions existed at a high concentration, the introduction of an oxygen-containing gas such as air was still difficult to prevent polymerization so that popcorn polymers tended to adhere in the heating unit and the reduced-pressure evaporation unit.
As has been described above, the method featuring introduction of an oxygen-containing gas such as air as a polymerization inhibitor in a concentration apparatus instead of causing a polymerization inhibitor such as copper ions to exist at a high concentration have high tendency to induce adhering of popcorn polymers on walls of processing units such as heating unit, evaporation unit and fractionation unit. This adhering problem therefore still remains unresolved.